An image sensor may be a semiconductor device that may convert an optical image into an electrical signal. An image sensor may be classified into categories, such as a charge coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor (CIS).
During a fabrication process of an image sensor, a photodiode may be formed in a substrate using ion implantation. A size of a photodiode may be reduced to increase a number of pixels without increasing a chip size. This may reduce an area of a light receiving portion. Image quality may thereby be reduced.
Since a stack height may not reduce as much as a reduction in an area of a light receiving portion, a number of photons incident to a light receiving portion may also be reduced due to diffraction of light called Airy disk.
To address this limitation, a photodiode may be formed using amorphous silicon (Si). In addition, readout circuitry may be formed in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and a photodiode may be formed on and/or over readout circuitry (referred to as a three-dimensional (3D) image sensor). A photodiode may be connected with readout circuitry through a metal interconnection.
According to the related art, it may be difficult to electrically connect a photodiode to readout circuitry. That is, a metal interconnection may be formed on and/or over readout circuitry and wafer-to-wafer bonding may be performed such that a metal interconnection may contact the photodiode. Hence, a contact between a metal interconnection may be difficult, and an ohmic contact between a metal interconnection and a photodiode may be difficult.
After wafer-to-wafer bonding may be performed, an etching process may be performed and may separate a photodiode for each unit pixel. An interface of a photodiode may be damaged and a dangling bond and the like may occur. This may act as a dark current source.
Since both a source and a drain at both sides of a transfer transistor may be heavily doped with N−type impurities, a charge sharing phenomenon may occur. When a charge sharing phenomenon occurs, a sensitivity of an output image may be reduced and an image error may be generated. In addition, because a photo charge may not readily move between a photodiode and readout circuitry, a dark current may be generated and/or saturation and sensitivity may be reduced.